1. Field of the Invention
The present invention relates to an ink set comprising in combination an ink and a reactive liquid containing a component reacting with a component contained in the ink and an image forming process, which are suitable for use in ink-jet printers, and a water-based ink used therein, and particularly to an ink set and an image forming process, which are useful for plain paper, and a water-based ink used therein.
2. Related Background Art
Ink-jet recording is a printing method wherein minute droplets of an ink are ejected to apply them to a recording medium such as paper, thereby conducting printing, and wherein high-resolution and high-quality images can be printed at high speed by an inexpensive apparatus and has been commonly used as a way of simply forming high-quality color images in recent years. There is, however, a demand for formation of higher-quality color images. On the other hand, various proposals have heretofore been made as to devices for forming higher-quality color images. For example, it has been proposed to use a black ink having characteristics of thickening or aggregating by an action with a salt and a color ink containing the salt in combination, thereby providing high-quality color images high in image density and free from occurrence of bleeding (color mixing between different colors) (see Japanese Patent Application Laid-Open No. H6-106735). In other words, according to this method, two inks different in properties from each other, i.e., the color ink containing the salt, that is the second liquid, and the black ink that is the first liquid, are used to form a print, whereby components in these two liquids are caused to react to form aggregates of coloring materials, thereby permitting formation of good-quality images. More specifically, a salt of a polyvalent metal ion is used as the salt.
Various proposals have also been made on formation of high-quality color images by using two liquids different in properties from each other, i.e., a set composed of a combination of a liquid composition containing no coloring material and an ink (see Japanese Patent Application Laid-Open Nos. H9-207424 and 2000-37942). As the liquid composition used in combination with the ink, for example, proposals have been made on liquid compositions containing a polyvalent metal salt having excellent reactivity to an anionic coloring material to easily form an aggregate of the coloring material and additionally a nonionic resin (see Japanese Patent Publication No. H6-86142 and Japanese Patent Application Laid-Open No. 2000-94825).
The present inventors have carried out a further extensive investigation as to the ink sets containing the polyvalent metal salt among these proposals. As a result, it has been confirmed that printing is conducted by such a printing method as disclosed in Japanese Patent Application Laid-Open No. H6-106735, whereby high-quality color images high in image density and free from occurrence of bleeding can be provided.
On the other hand, it has also been known to add a substance for increasing binding property between an aggregate of a coloring material and the surface of paper or between such aggregates, i.e., a substance having binding ability to an ink to improve the rub-off resistance of the resulting image (see Japanese Patent Publication No. H6-86142 and Japanese Patent Application Laid-Open No. 2000-94825). There have further been known a technique that a reactive liquid containing a cationic resin is applied to a recording medium to aggregate a component in an ink (Japanese Patent Application Laid-Open No. 2000-281947), a technique that a component for further hardening a resin in an ink-receiving layer that is dried and solidified is contained in an ink (Japanese Patent Application Laid-Open No. 2000-1641), and a technique that a substance for thickening a high-molecular compound in an ink is applied to a recording medium (Japanese Patent Application Laid-Open No. HI 1-78211), and the like.